The field of the present invention is structural designs for internal combustion engine crankcases.
Crankcases for internal combustion engines typically have been formed as part of the engine block with the underside of the block machined to receive bearings supporting the crankcase of the engine. The bearings are then held in place by bearing caps rigidly bolted to the underside of the engine block. Certain designs have contemplated the employment of a lower case as a means for supporting the bearings against the underside of the engine block. In such an instance, bolts extend upwardly through the lower case and into the engine block for rigidly affixing the lower case in position. With such a design, this rigid fixing of the lower case with the engine block results in a secure mounting for the main bearings in bearing seats defined between the block and the lower case.
In relatively small engines, such as small motorcycle engines, designs have been developed which employ a crankcase construction divided at a plane perpendicular to the axis of the crankshaft. Thus, the bearing walls supporting the crankshaft are unitary in construction. Seats for the bearings are created in such bearing walls. With such crankcase designs, significant loading on the crankshaft of such engines must be resisted by the unitary bearing wall as a result of the inertial forces associated with the reciprocal motion of the piston or pistons. Such loading is specifically imposed on the bearing walls of the crankcase about the crankshaft in a direction substantially parallel to the centerline of the cylinder or cylinders. As such, the loading is at a maximum through a cross-section of each bearing wall perpendicular to the line of force which is substantially parallel to the centerline of the cylinder or cylinders.
An obvious solution to the problem of excessive load in the aforementioned area in crankcases split along a plane perpendicular to the crankshaft is to increase the wall thickness of the crankcase in the areas about the bearing holes experiencing excessive stress. However, such a solution is not always practical. With existing crankcase designs, it may be impossible to increase the wall thickness surrounding the bearing hole because of interference with other components. Even in new designs, little room is often available for increased bearing length without like increases in engine length. Such solutions are near impossible with existing engines needing reinforcement.
Another obvious solution is to employ high strength material in fabrication of the crankcase. However, lighter engines are preferred, preferably in such applications as motorcycles and the like. As a result, die cast aluminum has become widely used for the fabrication of crankcases. Substantial weight advantage would be lost through employment of cast iron or the like.